1. Field of the Invention
The present invention relates to an analog semiconductor integrated circuit device having a sensor or a power management function.
2. Description of the Related Art
When an analog semiconductor integrated circuit device having a sensor or a power management function is composed of MOS transistors, a so called multi-Vth method, in which plural threshold voltages (hereinafter, referred to as Vth) of the MOS transistor, instead of one, are used, is generally applied in order to carry out complicated analog signal processing and to cover various input voltage ranges.
In a conventional semiconductor integrated circuit device as shown in FIGS. 5A and 5B, Vth value of a MOS transistor is determined by ion implantations 203 and 205. A plurality of photolithography steps and a plurality of ion implanting steps are performed to form a multi-Vth device (For example., refer to JP 2000-323587 A (p. 6, FIG. 2)).
In the prior art, a plurality of photolithography steps and a plurality of ion implanting steps to attain a multi-Vth device have increased the cost for manufacturing and have prolonged a manufacturing period, which results in a problem with product delivery. Further, since the number of steps cannot be increased unnecessarily, it is essentially impossible to attain multi-Vth having sufficiently many threshold voltages.